Power transmission device for a four-wheel drive vehicle

ABSTRACT

A torque transfer mechanism is provided for controlling the magnitude of a clutch engagement force exerted on a multi-plate clutch assembly that is operably disposed between a first rotary and a second rotary member. The torque transfer mechanism includes an actuator having a first segment fixed for rotation with the first rotary member and a second segment having a screw thread formed thereon which is rotatably and slidably disposed within a chamber filled with magnetorheological fluid. An electromagnetic coil is disposed in proximity to the chamber and is selectively energized for varying the viscosity of the magnetorheological fluid so as to induce axial movement of the actuator for engaging the multi-plate clutch assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/357,186 filed on Feb. 3, 2003 now U.S. Pat. No. 6,755,290.

FIELD OF THE INVENTION

The present invention relates generally to power transfer systems forcontrolling the distribution of drive torque between the front and reardrivelines of a four-wheel drive vehicle. More particularly, the presentinvention is directed to a power transmission device for use in motorvehicle driveline applications and having a magnetorheological clutchactuator that is operable for controlling actuation of a multi-platefriction clutch assembly.

BACKGROUND OF THE INVENTION

In view of increased demand for four-wheel drive vehicles, a plethora ofpower transfer systems are currently being incorporated into vehiculardriveline applications for transferring drive torque to the wheels. Inmany vehicles, a power transmission device is operably installed betweenthe primary and secondary drivelines. Such power transmission devicesare typically equipped with a torque transfer mechanism for selectivelyand/or automatically transferring drive torque from the primarydriveline to the secondary driveline to establish a four-wheel drivemode of operation. For example, the torque transfer mechanism caninclude a dog-type lock-up clutch that can be selectively engaged forrigidly coupling the secondary driveline to the primary driveline toestablish a “part-time” four-wheel drive mode. In contrast, drive torqueis only delivered to the primary driveline when the lock-up clutch isreleased for establishing a two-wheel drive mode.

A modern trend in four-wheel drive motor vehicles is to equip the powertransmission device with an adaptive transfer clutch in place of thelock-up clutch. The transfer clutch is operable for automaticallydirecting drive torque to the secondary wheels, without any input oraction on the part of the vehicle operator, when traction is lost at theprimary wheels for establishing an “on-demand” four-wheel drive mode.Typically, the transfer clutch includes a multi-plate clutch assemblythat is installed between the primary and secondary drivelines and aclutch actuator for generating a clutch engagement force that is appliedto the clutch plate assembly. The clutch actuator can be apower-operated device that is actuated in response to the magnitude ofan electric control signal sent from an electronic controller unit(ECU). Variable control of the control signal is typically based onchanges in current operating characteristics of the vehicle (i.e.,vehicle speed, interaxle speed difference, acceleration, steering angle,etc.) as detected by various sensors. Thus, such “on-demand” powertransmission devices can utilize adaptive control schemes forautomatically controlling torque distribution during all types ofdriving and road conditions.

Currently, a large number of on-demand transfer cases are equipped withan electrically-controlled clutch actuator that can regulate the amountof drive torque transferred to the secondary output shaft as a functionof the value of the electrical control signal applied thereto. In someapplications, the transfer clutch employs an electromagnetic clutch asthe power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024discloses a electromagnetic coil that is incrementally activated tocontrol movement of a ball-ramp drive assembly for applying a clutchengagement force on the multi-plate clutch assembly. Likewise, JapaneseLaid-open Patent Application No. 62-18117 discloses a transfer clutchequipped with an electromagnetic actuator for directly controllingactuation of the multi-plate clutch pack assembly.

As an alternative, the transfer clutch can employ an electric motor anda drive assembly as the power-operated clutch actuator. For example,U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having atransfer clutch equipped with an electric motor that controls rotationof a sector plate which, in turn, controls pivotal movement of a leverarm that is operable for applying the clutch engagement force to themulti-plate clutch assembly. Moreover, Japanese Laid-open PatentApplication No. 63-66927 discloses a transfer clutch which uses anelectric motor to rotate one cam plate of a ball-ramp operator forengaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos.4,895,236 and 5,423,235 respectively disclose a transfer case equippedwith a transfer clutch having an electric motor driving a reductiongearset for controlling movement of a ball screw operator and aball-ramp operator which, in turn, apply the clutch engagement force tothe clutch pack.

While many on-demand clutch control systems similar to those describedabove are currently used in four-wheel drive vehicles, a need exists toadvance the technology and address recognized system limitations. Forexample, the size, weight and electrical power requirements of theelectromagnetic coil or the electric motors needed to provide thedescribed clutch engagement loads may make such system cost prohibitivein some four-wheel drive vehicle applications. In an effort to addressthese concerns, new technologies are being considered for use inpower-operated clutch actuator applications such as, for example,magnetorheological clutch actuators. Examples of such an arrangement aredescribed in U.S. Pat. Nos. 5,915,513 and 6,412,618 wherein amagnetorheological actuator controls operation of a ball-ramp unit toengage the clutch pack. While such an arrangement may appear to advancethe art, its complexity clearly illustrates the need to continuedevelopment of even further defined advancement.

SUMMARY OF THE INVENTION

Thus, its is an object of the present invention to provide a powertransmission device for use in a motor vehicle having a torque transfermechanism equipped with a magnetorheological clutch actuator that isoperable to control engagement of a multi-plate clutch assembly.

It is a further object of the present invention to provide amagnetorheological screw pump for use as the clutch actuator in a torquetransfer mechanism.

As a related object, the torque transfer mechanism of the presentinvention is well-suited for use in motor vehicle driveline applicationsto control the transfer of drive torque between a first rotary memberand a second rotary member.

According to a preferred embodiment, the torque transfer mechanismincludes a magnetorheological clutch actuator which is operable forcontrolling the magnitude of clutch engagement force exerted on amulti-plate clutch assembly that is operably disposed between the firstrotary member and a second rotary member. The magnetorheological clutchactuator includes a threaded screw cam that is splined for rotation withthe first rotary member and disposed within a sealed chamber filled witha magnetorheological fluid. The magnetorheological clutch actuatorfurther includes an electromagnetic coil which surrounds a portion ofthe sealed fluid chamber. In operation, activation of theelectromagnetic coil creates a magnetic flux field which travels throughthe magnetorheological fluid for proportionally increasing itsviscosity, thereby creating drag between the screw cam and the firstrotary member which, in turn, causes axial movement of the screw cam.Such axial movement causes the screw cam to push against a pressureplate and exert a clutch engagement force on the clutch pack fortransferring torque from the first rotary member to the second rotarymember. Upon deactivation of the electromagnetic coil, a return springreleases the clutch pack from engagement and acts to axially move thescrew cam to a neutral position.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbecome apparent to those skilled in the art from analysis of thefollowing written description, the appended claims, and accompanyingdrawings in which:

FIG. 1 illustrates the drivetrain of a four-wheel drive vehicle equippedwith a power transmission device incorporating the present invention;

FIG. 2 is a schematic illustration of an on-demand 4WD transfer caseequipped with a torque transfer mechanism having a magnetorheologicalclutch actuator and a multi-plate friction clutch;

FIG. 3 is a partial sectional view of the transfer case showing thetorque transfer mechanism arranged for selectively transferring drivetorque from the primary output shaft to the secondary output shaft;

FIG. 4 is a partial sectional view of an alternative embodiment of thetorque transfer mechanism arranged for selectively transferring drivetorque from the primary output shaft to the secondary output shaft;

FIGS. 5 and 6 are partial sectional views of further alternativeembodiments of the torque transfer mechanism according to the presentinvention;

FIG. 7 is a schematic illustration of an alternative drivetrain for afour-wheel drive vehicle equipped with a power transmission device ofthe present invention; and

FIGS. 8 through 11 are schematic illustrations of alternativeembodiments of power transmission devices according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a torque transfer mechanism thatcan be adaptively controlled for modulating the torque transferred froma first rotary member to a second rotary member. The torque transfermechanism finds particular application in power transmission devices foruse in motor vehicle drivelines such as, for example, an on-demandclutch in a transfer case or in-line torque coupling, a biasing clutchassociated with a differential assembly in a transfer case or a driveaxle assembly, or as a shift clutch in a multi-speed automatictransmission. Thus, while the present invention is hereinafter describedin association with particular arrangements for use in a specificdriveline application, it will be understood that theconstruction/application shown and described is merely intended toillustrate embodiments of the present invention.

With particular reference to FIG. 1 of the drawings, a drivetrain 10 fora four-wheel drive vehicle is shown. Drivetrain 10 includes a primarydriveline 12, a secondary driveline 14, and a powertrain 16 fordelivering rotary tractive power (i.e., drive torque) to the drivelines.In the particular arrangement shown, primary driveline 12 is the reardriveline while secondary driveline 14 is the front driveline.Powertrain 16 includes an engine 18, a multi-speed transmission 20, anda power transmission device hereinafter referred to as transfer case 22.Rear driveline 12 includes a pair of rear wheels 24 connected atopposite ends of a rear axle assembly 26 having a rear differential 28coupled to one end of a rear prop shaft 30, the opposite end of which iscoupled to a rear output shaft 32 of transfer case 22. Front driveline14 includes a pair of front wheels 34 connected at opposite ends of afront axle assembly 36 having a front differential 38 coupled to one endof a front prop shaft 40, the opposite end of which is coupled to afront prop shaft 42 of transfer case 22.

With continued reference to the drawings, drivetrain 10 is shown tofurther include an electronically-controlled power transfer system forpermitting a vehicle operator to select between a two-wheel high-rangedrive mode, a part-time four-wheel high-range drive mode, an on-demandfour-wheel high-range drive mode, a neutral non-driven mode, and apart-time four-wheel low-range drive mode. In this regard, transfer case22 is equipped with a range clutch 44 that is operable for establishingthe high-range and low-range drive connections between an input shaft 46and rear output shaft 32, and a power-operated range actuator 48 that isoperable to actuate range clutch 44. Transfer case 22 also a transferclutch 50 that is operable for transferring drive torque from rearoutput shaft 32 to front output shaft 42 for establishing the part-timeand on-demand four-wheel drive modes. The power transfer system furtherincludes a power-operated mode actuator 52 for actuating transfer clutch50, vehicle sensors 54 for detecting certain dynamic and operationalcharacteristics of the motor vehicle, a mode select mechanism 56 forpermitting the vehicle operator to select one of the available drivemodes, and a controller 58 for controlling actuation of range actuator48 and mode actuator 52 in response to input signals from vehiclesensors 54 and mode selector 56.

Transfer case 22 is shown schematically in FIG. 2 to include a housing60 from which input shaft 46 is rotatably supported by a bearingassembly 62. As is conventional, input shaft 46 is adapted for drivenconnection to the output shaft of transmission 20. Rear output shaft 32is shown rotatably supported between input shaft 46 and housing 60 viabearing assemblies 64 and 66 while front output shaft 42 is rotatablysupported between transfer mechanism 50 and housing 60 by a pair oflaterally-spaced bearing assemblies 68 and 69. Range clutch 44 is shownto include a planetary gearset 70 and a synchronized range shiftmechanism 72. Planetary gearset 70 includes a sun gear 74 fixed forrotation with input shaft 46, a ring gear 76 fixed to housing 60, and aset of planet gears 78 rotatably supported on pinion shafts 80 extendingbetween front and rear carrier rings 82 and 84, respectively, that areinterconnected to define a carrier 86.

Planetary gearset 70 functions as a two-speed reduction unit which, inconjunction with a sliding range sleeve 88 of synchronized range shiftmechanism 72, is operable to establish either of a first or second driveconnection between input shaft 46 and rear output shaft 32. To establishthe first drive connection, input shaft 46 is directly coupled to rearoutput shaft 32 for defining a high-range drive mode in which rearoutput shaft 32 is driven at a first (i.e., direct) speed ratio relativeto input shaft 46. Likewise, the second drive connection is establishedby coupling carrier 86 to rear output shaft 32 for defining a low-rangedrive mode in which rear output shaft 32 is driven at a second (i.e.,reduced) speed ratio relative to input shaft 46. A neutral non-drivenmode is established when rear output shaft 32 is disconnected from bothinput shaft 46 and carrier 86.

Synchronized range shift mechanism 72 includes a first clutch plate 90fixed for rotation with input shaft 46, a second clutch plate 92 fixedfor rotation with rear carrier ring 84, a clutch hub 94 rotatablysupported on input shaft 46 between clutch plates 90 and 92, and a driveplate 96 fixed for rotation with rear output shaft 32. Range sleeve 88has a first set of internal spline teeth that are shown meshed withexternal spline teeth on clutch hub 94, and a second set of internalspline teeth that are shown meshed with external spline teeth on driveplate 96. As will be detailed, range sleeve 88 is axially moveablebetween three distinct positions to establish the high-range, low-rangeand neutral modes. Range shift mechanism 72 also includes a firstsynchronizer assembly 98 located between hub 94 and first clutch plate90, and a second synchronizer assembly 100 disposed between hub 94 andsecond clutch plate 92. Synchronizers 98 and 100 work in conjunctionwith range sleeve 88 to permit on-the-move range shifts.

With range sleeve 88 located in its neutral position, as denoted byposition line “N”, its first set of spline teeth are disengaged from theexternal clutch teeth on first clutch plate 90 and from the externalclutch teeth on second clutch plate 92. Thus, no drive torque istransferred from input shaft 46 to rear output shaft 32 when rangesleeve 88 is in its neutral position. When it is desired to establishthe high-range drive mode, range sleeve 88 is slid axially from itsneutral position toward a high-range position, denoted by position line“H”. First synchronizer assembly 98 is operable for causing speedsynchronization between input shaft 46 and rear output shaft 32 inresponse to sliding movement of range sleeve 88 from its neutralposition toward its high-range position. Upon completion of speedsynchronization, the first set of spline teeth on range sleeve 88 moveinto meshed engagement with the external clutch teeth on first clutchplate 90 while its second set of spline teeth are maintained inengagement with the spline teeth on drive plate 96. Thus, movement ofrange sleeve 88 to its high-range position acts to couple rear outputshaft 32 for common rotation with input shaft 46 and establishes thehigh-range drive mode connection therebetween. Similarly, secondsynchronizer assembly 100 is operable for causing speed synchronizationbetween carrier 86 and rear output shaft 32 in response to axial slidingmovement of range sleeve 88 from its neutral position toward a low-rangeposition, as denoted by position line “L”. Upon completion of speedsynchronization, the first set of spline teeth on range sleeve 88 moveinto meshed engagement with the external clutch teeth on second clutchplate 92 while the second set of spline teeth on range sleeve 88 aremaintained in engagement with the external spline teeth on drive plate96. Thus, with range sleeve 88 located in its low-range position, rearoutput shaft 32 is coupled for rotation with carrier 86 and thelow-range drive mode connection is established between input shaft 46and rear output shaft 32.

To provide means for moving range sleeve 88 between its three distinctrange position, range shift mechanism 72 further includes a range fork102 coupled to range sleeve 88. Range actuator 48 is operable to moverange fork 102 for causing corresponding axial movement of range sleeve88 between its three range positions. Range actuator 48 is preferably anelectric motor arranged to move range sleeve 88 to a specific rangeposition in response to a control signal from controller 58 that isbased on the mode signal delivered to controller 58 from mode selectmechanism 56.

It will be appreciated that the synchronized range shift mechanismpermits “on-the-move” range shifts without the need to stop the vehiclewhich is considered to be a desirable feature. However, othersynchronized and non-synchronized versions of range clutch 44 can beused in substitution for the particular arrangement shown. Also, it iscontemplated that range clutch 44 and range actuator 48 can be removedentirely from transfer case 22 such that input shaft 46 would directlydrive rear output shaft 32 to define a one-speed version of theon-demand transfer case embodying the present invention.

Referring now primarily to FIGS. 2 and 3, transfer clutch 50 is shownarranged in association with front output shaft 42 in such a way that itfunctions to deliver drive torque from a transfer assembly 110 driven byrear output shaft 32 to front output shaft 42 for establishing thefour-wheel drive modes. Transfer assembly 110 includes a first sprocket112 fixed for rotation with rear output shaft 32, a second sprocket 114rotatably supported by bearings 116 on front output shaft 42, and apower chain 118 encircling sprockets 112 and 114. As will be detailed,transfer clutch 50 is a multi-plate clutch assembly 124 and modeactuator 52 is a magnetorheological clutch actuator 120 which togetherdefine a torque transfer mechanism.

Multi-plate clutch assembly 124 is shown to include an annular drum 126fixed for rotation with second sprocket 114, a hub 128 fixed via asplined connection 130 for rotation with front output shaft 42, and amulti-plate clutch pack 132 operably disposed between drum 126 and hub128. In particular, drum 126 has a first smaller diameter cylindricalrim 126A that is fixed (i.e., welded, splined, etc.) to sprocket 114 anda second larger diameter cylindrical rim 126B that is interconnected torim 126A by a radial plate segment 126C. Hub 128 is shown to include afirst smaller diameter hub segment 128A and a second larger diameter hubsegment 128B that are interconnected by a radial plate segment 128C.Clutch pack 132 includes a set of outer friction plates 134 that aresplined to outer rim 126B of drum 126 and which are alternativelyinterleaved with a set of inner friction plates 136 that are splined tohub segment 128B of clutch hub 128. Clutch assembly 124 further includesa first pressure plate 138 having a plurality ofcircumferentially-spaced and radially-extending tangs 140 that aredisposed in longitudinally-extending slots formed in hub segment 128Bprior to installation of clutch pack 132 such that a front face surface142 of each tang 140 abuts an end surface 144 of the slots so as todefine a fully retracted position of first pressure plate 138 relativeto clutch pack 132. Thus, first pressure plate 138 is coupled for commonrotation with clutch hub 128 and front output shaft 42. A secondpressure plate 146 is fixed via a splined connection 147 to rim 126B ofdrum 126 for rotation therewith. As seen, a plurality ofcircumferentially-spaced return springs 148 act between pressure plates138 and 146.

With continued reference to FIGS. 2 and 3, magnetorheological clutchactuator 120 is shown to generally include a screw cam 150 and anelectromagnetic coil 152. Screw cam 150 has a flange segment 162 fixedvia a splined connection 164 for rotation with drum 126, and a frontface surface 166 in engagement with second pressure plate 146. Screw cam150 has an outer cylindrical surface 167 having a thread form 168 whichis disposed within a chamber 170 formed by housing 60. Thread form 168may have any suitable configuration of thread profiles (worm, helical,etc.) and pitch angles and should be sized to provide a small clearancebetween housing 60 and outer surface 167 of screw cam 150. In addition,the hand (i.e., left or right) orientation of thread form 168 isselected to inhibit axial movement of screw cam 150 toward clutch pack132 due to fluid pumping action caused by rotation of screw cam 150 withdrum 126. Electromagnetic coil 152 is rigidly mounted to housing 60 andis shown to surround a portion of thread form 168. Coil 152 is arrangedto receive an electric control signal from controller 58. Screw cam 150is rotatably supported on a support 174 associated with housing 60 via abearing assembly 176. As seen, an end of front output shaft 42 isrotatably supported by bearing assembly 69 within an annular recess 178formed in screw cam 150.

Chamber 170 is sealed relative to screw cam 150 via suitable seal rings179 and includes an annular reservoir portion 180 that is filled with amagnetorheological (MR) fluid 182, preferably of a high viscosity and ofthe type manufactured by the Lord Corporation, Erie, Pa. In the absenceof a magnetic field (as generated via activation of electromagnetic coil152 as described herein), screw cam 150 acts as a screw pump for cyclingMR fluid 182 through chamber 170. However, when MR fluid 182 is exposedto a magnetic field, its magnetic particles align with the field andincrease the viscosity and, therefore, the shear strength of MR fluid182. Increased shear strength results in greater resistance to relativemotion of thread form 168 relative to housing 60. As will be understood,when the magnitude of the electric current sent to coil 152 bycontroller 58 exceeds a predetermined minimum value, the magnetic fieldpassing through MR fluid 182 causes a viscosity change sufficient toincrease the shear force acting on thread form 168. When this occurs,the frictional drag generated induces screw cam 150 to move axiallytoward clutch pack 132. Such axial movement of screw cam 150 causescorresponding movement of second pressure plate 146, in opposition tothe biasing force of springs 148, into engagement with clutch pack 132.

The biasing force of springs 148 limits axial movement of screw cam 150as a function of the viscosity of MR fluid 182. For example, in itsleast viscous form, MR fluid 182 has no effect and is simply pumped bythread form 168 within chamber 170. In its most viscous form, MR fluid182 enables the thread interface between screw cam 150 and housing 60for inducing sufficient axial movement of screw cam 150 to fully engageclutch pack 132. However, axial movement of screw cam 150 is limited atfull engagement of clutch pack 132 and once having achieved that limit,screw cam 150 continues to rotate with drum 126, while still pumping thenow highly viscous MR fluid 182 within chamber 170. Degrees of viscosityare achievable between the least viscous and most viscous form of MRfluid 182 and vary with the intensity of the magnetic field and, thus,with the magnitude of the electric control signal sent to coil 152. Assuch, the value of the clutch engagement force induced by screw cam 150and applied to clutch pack 132 of clutch assembly 124 can be adaptivelyvaried as a function of the magnitude of the electric control signalsent to coil 152 between a no torque transfer condition (two-wheel drivemode with 100% of drive torque to rear output shaft 32) and atorque-split condition (part-time four-wheel drive mode with 50% ofdrive torque to front output shaft 42 and 50% to rear output shaft 32).Upon decease of the magnetic field strength, screw cam 150 is axialbiased by springs 148 against second pressure plate 146, therebyrelieving engagement of clutch pack 132 and moving screw cam 150 to itsreleased position.

In operation, when mode selector 56 indicates selection of the two-wheelhigh-range drive mode, range actuator 48 is signaled to move rangesleeve 88 to its high-range position and transfer clutch 50 ismaintained in a released condition with no electric signal sent to coil150 of magnetorheological clutch actuator 120, whereby all drive torqueis delivered to rear output shaft 32. If mode selector 56 thereafterindicates selection of a part-time four-wheel high-range mode, rangesleeve 88 is maintained in its high-range position and a predeterminedmaximum electrical control signal is sent by controller 58 to coil 152of magnetorheological clutch actuator 120 which causes axial movement ofscrew cam 150 due to the resultant change in viscosity of MR fluid 182.Such action causes second pressure plate 146 to engage clutch pack 132until a maximum clutch engagement force is exerted on clutch pack 132for effectively coupling hub 128 to drum 126. In response to suchmovement of second pressure plate 146, return springs 148 are compressedand act to forcibly locate first pressure plate 138 in its fullyretracted position where it acts as a reaction plate against whichclutch pack 132 is compressed.

If a part-time four-wheel low-range drive mode is selected, theoperation of transfer clutch 50 and magnetorheological clutch actuator120 are identical to that described above for the part-time high-rangedrive mode. However, in this mode, range actuator 48 is signaled tolocate range sleeve 88 in its low-range position to establish thelow-range drive connection between input shaft 46 and rear output shaft32.

When the mode signal indicates selection of the on-demand four-wheelhigh-range drive mode, range actuator 48 moves or maintains range sleeve88 in its high-range position and magnetorheological clutch actuator 120is placed in a ready or “stand-by” condition. Specifically, the minimumamount of drive torque sent to front output shaft 42 through transferclutch 50 in the stand-by condition can be zero or a slight amount(i.e., in the range of 2-10%) as required for the certain vehicularapplication. This minimum stand-by torque transfer is generated bycontroller 58 sending a control signal to coil 152 having apredetermined minimum value. Thereafter, controller 58 determines whenand how much drive torque needs to be transferred to front output shaft42 based on tractive conditions and/or vehicle operating characteristicsdetected by vehicle sensors 54. For example, FIG. 2 illustrates a firstspeed sensor 212 which sends a signal to controller 58 indicative of therotary speed of rear output shaft 32 while a second speed sensor 214sends a signal indicative of the rotary speed of front output shaft 42.Controller 58 can vary the value of the electric control signal sent tocoil 152 between the predetermined minimum value and the predeterminedmaximum value based on defined relationships such as, for example, thespeed difference ΔRPM between shafts 32 and 42.

With particular reference now to FIG. 4, an alternative embodiment of atorque transfer mechanism is shown to include multi-plate clutchassembly 124 and a modified magnetorheological clutch actuator 120A. Indescribing the alternative embodiment, it will be appreciated thatcommon reference numerals indicate similar components. In general,clutch actuator 120A is similar to clutch actuator 120 with theexception that thread form 168A is formed on an internal cylindricalsurface 190 of screw cam 150A and electromagnetic coil 152A is nowmounted in an annular casing 192 that is secured to housing 60 oftransfer case 22. Also, reservoir 180A is formed in casing 192 such thatit communicates with a chamber 170A formed between casing 192 and threadform 168A. It will be understood that clutch actuator 120A functionssimilarly to that of clutch actuator 120 in that the magnitude of theelectric current sent to coil 152A functions to control the viscosity ofMR fluid 182 in chamber 170A and thus, the amount of drag generatedbetween casing 192 and screw cam 150A. In this manner, screw cam 150Arotates with rear output shaft 32 (via transfer assembly 110 and drum126) and is axially moveable relative thereto for adaptively engagingclutch assembly 124.

While the torque transfer mechanism is shown arranged on front outputshaft 42, it is evident that it could easily be installed on rear outputshaft 32 for transferring drive torque to a transfer assembly arrangedto drive front output shaft 42. Furthermore, the present invention canbe used as a torque transfer coupling in an all-wheel drive (AWD)vehicle to selectively and/or automatically transfer drive torqueon-demand from the primary (i.e., front) driveline to the secondary(i.e., rear) driveline. Likewise, in full-time transfer cases equippedwith an interaxle differential, torque transfer clutch 50 could be usedto limit slip and bias torque across the differential.

Referring now to FIG. 5, a torque transfer mechanism, hereinafterreferred to as transfer coupling 200, is shown to include a multi-plateclutch assembly 202 operably installed between an input member 204 andan output member 206, and a magnetorheological clutch actuator 208.Clutch assembly 202 includes a set of inner clutch plates 210 fixed viaa spline connection 212 for rotation with input member 204, a clutchdrum 214 fixed to output member 206, and a set of outer clutch plates216 fixed via a spline connection 218 to clutch drum 214. As seen, outerclutch plates 216 are alternatively interleaved with inner clutch plates210 to define a clutch pack. Drum 214 has a radial plate segment 220which functions as a reaction plate against which the interleaved clutchplates can be frictionally engaged. A bearing assembly 222 is shownsupporting drum 214 for rotation relative to input member 204.

With continued reference to FIG. 5, clutch actuator 208 is shown toinclude a screw cam 224 and an electromagnetic coil 226. Screw cam 224includes a plate segment 228, a first cylindrical rim segment 230, and asecond cylindrical rim segment 232. First rim segment 230 includesinternal spline teeth that are meshed with the external spline teeth oninput member 204 to define a spline connection 234 therebetween. Firstrim segment 230 also includes a thrust face surface 236 that is adaptedto engage a pressure plate 238 disposed between the clutch pack andscrew cam 224. Second rim segment 232 has an inner cylindrical surfacesupported by a bearing assembly 240 for rotation relative to a housing242. An outer cylindrical surface of second rim segment 232 has a threadform 244 which is sealed via seal rings 246 relative to housing 242 todefine a fluid chamber 248 therebetween. An annular reservoir 250 isformed in housing 242 and communicates with chamber 248. Reservoir 250and chamber 248 are filled with a volume of MR fluid 182.

Electromagnetic coil 226 is mounted in housing 242 and surrounds aportion of thread form 244 of screw cam 224. Coil 226 is adapted toreceived electric control signals from controller 58. The geometricconfiguration of thread form 244 is selected to provide a pumping actionin response to rotation of screw cam 224 caused by rotation of inputmember 204. As is similar to previously described magnetorheologicalclutch actuators 120 and 120A, clutch actuator 208 functions to controlaxial movement of screw cam 224 relative to input member 204 and clutchassembly 202 in proportion to the magnitude of the electric current sentto coil 226. As before, varying in the electric current sent to coil 226causes corresponding changes in the viscosity of MR fluid 182 which, inturn, causes relative rotation between cam 224 and input member 204 dueto increased drag. It is contemplated that transfer coupling 200 couldbe readily used in various driveline applications including, withoutlimitation, as the on-demand transfer clutch or the full-time biasclutch in 4WD transfer units, as an in-line coupling or power take-offunit, or as a limited slip coupling in drive axles and AWD systems.

Referring now to FIG. 6, a modified version of torque transfer coupling200 is designated by reference numeral 200A. Again, common referencenumbers are used to identify similar components. In essence, coupling200A is generally similar to coupling 200 with the exception that inputmember 204A is fixed via a spline connection 260 to second rim segment232 of screw cam 224A and spline connection 234A now couples screw cam224A for rotation with a clutch hub 262. As seen, clutch hub 262 isassociated with clutch assembly 202A such that inner clutch plates 210are fixed via a spline connection 212 to hub 262. A bearing assembly 264rotatably supports input member 204A relative to housing 242A. However,in all aspects of its operation, torque transfer coupling 200A issubstantially identical to coupling 200.

To illustrate an alternative power transmission device to which thepresent invention is applicable, FIG. 7 schematically depicts afront-wheel based four-wheel drivetrain layout 10′ N for a motorvehicle. In particular, engine 18 drives a multi-speed transmission 20′having an integrated front differential unit 38′ for driving frontwheels 34 via axle shafts 33. A transfer unit 35 is also driven bytransmission 20′ for delivering drive torque to the input member of anin-line torque transfer coupling 300 via a drive shaft 30′. Inparticular, the input member of transfer coupling 300 is coupled todrive shaft 30′ while its output member is coupled to a drive componentof rear differential 28. Accordingly, when sensors indicate theoccurrence of a front wheel slip condition, controller 58 adaptivelycontrols actuation of torque coupling 300 such that drive torque isdelivered “on-demand” to rear wheels 24. It is contemplated that torquetransfer coupling 300 would include a multi-plate transfer clutch and amagnetorheological clutch actuator that are generally similar instructure and function to that of any of the devices previouslydescribed herein. While shown in association with rear differential 28,it is contemplated that torque coupling 300 could also be operablylocated for transferring drive torque from transfer unit 35 to driveshaft 30′.

Referring now to FIG. 8, torque coupling 300 is schematicallyillustrated in association with an on-demand four-wheel drive systembased on a front-wheel drive vehicle similar to that shown in FIG. 7. Inparticular, an output shaft 302 of transaxle 20′ is shown to drive anoutput gear 304 which, in turn, drives an input gear 306 fixed to acarrier 308 associated with front differential unit 38′. To providedrive torque to front wheels 34, front differential unit 38′ includes apair of side gears 310 that are connected to front wheels 34 viaaxleshafts 33. Differential unit 38′ also includes pinions 312 that arerotatably supported on pinion shafts fixed to carrier 308 and which aremeshed with side gears 310. A transfer shaft 314 is provided to transferdrive torque from carrier 308 to a clutch hub 316 associated with amulti-pate clutch assembly 318. Clutch assembly 318 further includes adrum 320 and a clutch pack 322 having interleaved clutch plates operablyconnected between hub 316 and drum 320.

Transfer unit 35 is a right-angled drive mechanism including a ring gear324 fixed for rotation with drum 320 of clutch assembly 318 which ismeshed with a pinion gear 326 fixed for rotation with drive shaft 30′.As seen, a magnetorheological clutch actuator 328 is schematicallyillustrated for controlling actuation of clutch assembly 318. Accordingto the present invention, magnetorheological actuator 328 is similar toany one of the various magnetorheological clutch actuators previouslydescribed in that an electromagnetic coil is supplied with electriccurrent for changing the viscosity of a magnetorheological fluid which,in turn, functions to control translational movement of a rotary screwcam for engaging clutch pack 322. In operation, drive torque istransferred from the primary (i.e., front) driveline to the secondary(i.e., rear) driveline in accordance with the particular mode selectedby the vehicle operator via mode selector 56. For example, if theon-demand 4WD mode is selected, controller 58 modulates actuation ofmagnetorheological clutch actuator 328 in response to the vehicleoperating conditions detected by sensors 54 by varying the value of theelectric control signal sent to the electromagnetic coil. In thismanner, the level of clutch engagement and the amount of drive torquethat is transferred through clutch pack 322 to the rear drivelinethrough transfer unit 35 and drive shaft 30′ is adaptively controlled.Selection of a locked or part-time 4WD mode results in full engagementof clutch assembly 318 for rigidly coupling the front driveline to therear driveline. In some applications, the mode selector 56 may beeliminated such that only the on-demand 4WD mode is available so as tocontinuously provide adaptive traction control without input from thevehicle operator.

FIG. 9 illustrates a modified version of FIG. 8 wherein an on-demandfour-wheel drive system is shown based on a rear-wheel drive motorvehicle that is arranged to normally deliver drive torque to rear wheels24 while selectively transmitting drive torque to front wheels 34through torque coupling 300. In this arrangement, drive torque istransmitted directly from transmission output shaft 302 to transfer unit35 via a drive shaft 330 interconnecting input gear 306 to ring gear324. To provide drive torque to front wheels 34, torque coupling 300 isnow shown operably disposed between drive shaft 330 and transfer shaft314. In particular, clutch assembly 318 is arranged such that drum 320is driven with ring gear 324 by drive shaft 330. As such, actuation ofmagnetorheological clutch actuator 328 functions to transfer torque fromdrum 320 through clutch pack 322 to hub 316 which, in turn, drivescarrier 308 of front differential unit 38′ via transfer shaft 314.Again, the vehicle could be equipped with mode selector 56 to permitselection by the vehicle operator of either the adaptively controlledon-demand 4WD mode or the locked part-time 4WD mode. In vehicles withoutmode selector 56, the on-demand 4WD mode is the only mode available andwhich provides continuous adaptive traction control with input from thevehicle operator.

In addition to the on-demand 4WD systems shown previously, the powertransmission (magnetorheological clutch actuator and clutch assembly)technology of the present invention can likewise be used in full-time4WD systems to adaptively bias the torque distribution transmitted by acenter or “interaxle” differential unit to the front and reardrivelines. For example, FIG. 10 schematically illustrates a full-timefour-wheel drive system which is generally similar to the on-demandfour-wheel drive system shown in FIG. 9 with the exception that aninteraxle differential unit 340 is now operably installed betweencarrier 308 of front differential unit 38′ and transfer shaft 314. Inparticular, output gear 306 is fixed for rotation with a carrier 342 ofinteraxle differential 340 from which pinion gears 344 are rotatablysupported. A first side gear 346 is meshed with pinion gears 344 and isfixed for rotation with drive shaft 330 so as to be drivinglyinterconnected to the rear driveline through transfer unit 35. Likewise,a second side gear 348 is meshed with pinion gears 344 and is fixed forrotation with carrier 308 of front differential unit 38′ so as to bedrivingly interconnected to the front driveline. In operation, whensensor 54 detects a vehicle operating condition, such as excessiveinteraxle slip, controller 58 adaptively controls activation of theelectromagnetic coil associated with magnetorheological clutch actuator328 for controlling engagement of clutch assembly 318 and thus thetorque biasing between the front and rear driveline.

Referring now to FIG. 11, a full-time 4WD system is shown to include atransfer case 22′ equipped with an interaxle differential 350 between aninput shaft 46′ and output shafts 32′ and 42′. Differential 350 includesan input defined as a planet carrier 352, a first output defined as afirst sun gear 354, a second output defined as a second sun gear 356,and a gearset for permitting speed differentiation between first andsecond sun gears 354 and 356. The gearset includes meshed pairs of firstplanet gears 358 and second pinions 360 which are rotatably supported bycarrier 352. First planet gears 358 are shown to mesh with first sungear 354 while second planet gears 350 are meshed with second sun gear356. First sun gear 354 is fixed for rotation with rear output shaft 32′so as to transmit drive torque to rear driveline 12. To transmit drivetorque to front driveline 14, second sun gear 356 is coupled to atransfer assembly 110′ which includes a first sprocket 112′ rotatablysupported on rear output shaft 32′, a second sprocket 114′ fixed tofront output shaft 42′, and a power chain 118′.

Transfer case 22′ further includes a biasing clutch 50′ having amulti-plate clutch assembly 124′ and a mode actuator 52′ having amagnetorheological clutch actuator 120′. Clutch assembly 124′ includes adrum 126′ fixed for rotation with first sprocket 112′, a hub 128′ fixedfor rotation with rear output shaft 32′, and a multi-plate clutch pack132′operably disposed therebetween. Magnetorheological clutch actuator120′ includes an electromagnetic coil that can be energized forcontrolling the viscosity of the magnetorheological fluid forcontrolling movement of a screw cam relative to clutch pack 132′.

A number of preferred embodiments have been disclosed to provide thoseskilled in the art an understanding of the best mode currentlycontemplated for the operation and construction of the presentinvention. The invention being thus described, it will be obvious thatvarious modifications can be made without departing from the true spiritand scope of the invention, and all such modifications as would beconsidered by those skilled in the art are intended to be includedwithin the scope of the following claims.

1. A power transmission device comprising: a rotary input member adaptedto receive drive torque from a source of torque; a rotary output memberadapted to provide drive torque to an output device; a torque transfermechanism operable for transferring drive torque from said input memberto said output member, said torque transfer mechanism including a clutchassembly operably disposed between said input member and said outputmember, and a magnetorheological clutch actuator having a threaded screwcam disposed within a chamber filled with magnetorheological fluid andan electromagnetic coil arranged to vary the viscosity of the fluid insaid chamber in response to electric control signals; and a controllerfor generating said electric control signals.
 2. The power transmissionof claim 1 wherein said chamber is provided between a housing and saidthreaded screw cam, and wherein said electromagnetic coil is mounted tosaid housing.
 3. The power transmission of claim 1 wherein said clutchassembly includes an interleaved clutch pack having a first set ofclutch plates fixed for rotation with said input member and a second setof clutch plates fixed for rotation with said output member, and whereinaxial movement of said screw cam controls the magnitude of a clutchengagement force exerted on said clutch pack.
 4. The power transmissionof claim 1 wherein said input member is a first output shaft of atransfer case and said output member is a second output shaft of saidtransfer case.
 5. The power transmission of claim 1 wherein said inputmember is driven by a powertrain of a motor vehicle and said outputmember is connected to a differential of an axle assembly.
 6. The powertransmission of claim 1 wherein said controller establishes the value ofsaid electric control signal based on a rotary speed difference betweensaid input member and said output member, and wherein said controlsignal is operable to vary the viscosity of said magnetorheologicalfluid in said chamber for causing relative rotation between said inputmember and said screw cam which results in axial movement of said screwcam relative to said clutch assembly.
 7. A transfer case for use in amotor vehicle having a powertrain and first and second drivelines,comprising: a first shaft driven by the powertrain and adapted forconnection to the first driveline; a second shaft adapted for connectionto the second driveline; a torque transfer mechanism operable fortransferring drive torque from said first shaft to said second shaft,said torque transfer mechanism including an input member driven by saidfirst shaft, an output member driving said second shaft, a clutchassembly operably disposed between said input member and said outputmember, and a clutch actuator operable for applying a clutch engagementforce on said clutch assembly, said clutch actuator including a screwcam supported for axial movement relative to said input member andhaving a threaded segment disposed within a chamber filled with amagnetorheological fluid, and an electromagnet arranged to vary theviscosity of said fluid in said chamber in response to electric controlsignals; and a controller for generating said electric control signals.8. The transfer case of claim 7 wherein said chamber is provided betweena housing and said threaded segment of said screw cam, and wherein saidelectromagnet includes a coil that is mounted to said housing.
 9. Thetransfer case of claim 7 wherein said clutch assembly includes aninterleaved clutch pack having a first set of clutch plates fixed forrotation with said input member and a second set of clutch plates fixedfor rotation with said output member, and wherein axial movement of saidscrew cam controls the magnitude of said clutch engagement force exertedon said clutch pack.
 10. The transfer case of claim 7 wherein saidcontroller establishes the value of said electric control signal basedon a rotary speed difference between said input member and said outputmember, and wherein said control signal is operable to vary theviscosity of said magnetorheological fluid in said chamber for causingrelative rotation between said input member and said screw cam whichresults in axial movement of said screw cam relative to said clutchassembly.
 11. A torque transfer mechanism for controlling the magnitudeof a clutch engagement force exerted on a clutch pack that is operablydisposed between a first rotary member and a second rotary member,comprising: an actuator having a screw thread formed thereon, saidactuator slidably and rotatably disposed adjacent a chamber filled witha magnetorheological fluid, said screw thread reacting against saidmagnetorheological fluid within said chamber; and an electromagnetadjacent to said chamber, wherein said electromagnet is selectivelyenergized for varying the viscosity of said magnetorheological fluid tovary a reaction force between said screw thread and saidmagnetorheological fluid so as to induce axial movement of said actuatorfor engaging the clutch pack.
 12. The torque transfer mechanism of claim11 wherein the magnitude of an engagement force generated by saidactuator and applied to said clutch pack is a function of said viscosityof said magnetorheological fluid.
 13. A torque transfer mechanism forcontrolling the magnitude of a clutch engagement force exerted on aclutch pack that is operably disposed between a first rotary member anda second rotary member, comprising: screw cam that is slidably androtatably disposed within a sealed chamber for pumping amagnetorheological fluid therein; and an electromagnet disposed adjacentsaid sealed chamber, said electromagnet can be selectively energized forvarying the viscosity of said magnetorheological fluid tocorrespondingly varying a pumping force, thereby inducing axial movementof said screw cam for engaging the clutch pack.
 14. The torque transfermechanism of claim 13 wherein the engagement force of said actuatorapplied to said clutch pack is a function of the viscosity of saidmagnetorheological fluid.
 15. In a transfer case having first and secondoutput shafts, a torque coupling for selectively coupling the firstoutput shaft to the second output shaft, comprising: a clutch assemblyoperably disposed between the first and second output shafts; and atransfer mechanism for controlling the magnitude of a clutch engagementforce exerted on said clutch assembly, said transfer mechanism includinga clutch actuator operable for selectively engaging said clutch assemblyand having a screw thread formed thereon, said clutch actuator isslidably and rotatably disposed adjacent a chamber filled with amagnetorheological fluid such that said screw thread reacts against saidmagnetorheological fluid within said chamber, and an electromagnetdisposed in close proximity to said chamber, said electromagnet can beselectively energized for varying the viscosity of saidmagnetorheological fluid to increase a reaction force between said screwthread and said magnetorheological fluid to induce axial movement ofsaid clutch actuator for engaging said clutch assembly.
 16. The torquecoupling of claim 15 wherein the engagement force of said clutchactuator applied to said clutch assembly is a function of the viscosityof said magnetorheological fluid.
 17. The torque coupling of claim 15further comprising a controller for selectively energizing saidelectromagnet to vary the viscosity of said magnetorheological fluid.18. A power transmission device comprising: a rotary input memberadapted to receive drive torque from a source of torque; a first rotaryoutput member adapted to provide drive torque to a first output device;a second rotary output member adapted to provide drive torque to asecond output device; a gearset operably interconnecting said inputmember to said first and second output members and permitting relativerotation therebetween; a torque transfer mechanism operable for limitingspeed differentiation between said first and second output members, saidtorque transfer mechanism including a friction clutch assembly disposedbetween said first output member and said second output member and aclutch actuator for applying a clutch engagement force to said frictionclutch assembly, said clutch actuator including a screw cam havingthreaded segment disposed within a chamber filled with amagnetorheological fluid and an electromagnetic coil arranged to varythe viscosity of said fluid in said chamber in response to an electriccontrol signal; and a controller for generating said electric controlsignal.
 19. The power transmission of claim 18 wherein said chamber isprovided between a housing and said threaded segment of said screw cam,and wherein said electromagnet includes a coil mounted to said housing.20. The power transmission of claim 18 wherein said input member is aninput shaft of a transfer case, said first output member is a firstoutput shaft of said transfer case, and said second output member is asecond output shaft of said transfer case, and wherein said gearset isan interaxle differential operably interconnecting said input shaft tosaid first and second output shafts.
 21. The power transmission of claim18 wherein said controller establishes the value of said electriccontrol signal based on a rotary speed difference between first andsecond output members, and wherein said control signal is operable tovary the viscosity of said magnetorheological fluid in said chamber forcausing relative rotation between said first output member and saidscrew cam which results in axial movement of said screw cam relative tosaid friction clutch assembly.
 22. A transfer case for use in a motorvehicle having a powertrain and first and second drivelines, comprising:an input shaft driven by the powertrain; a first output shaft adaptedfor connection to the first driveline; a second output shaft adapted forconnection to the second driveline; an interaxle differential operablyinterconnecting said input shaft to said first and second output shafts;a torque transfer mechanism operable for limiting speed differentiationbetween said first and second output shafts, said torque transfermechanism including a first member coupled to said first output shaft, asecond member coupled to second output shaft, a friction clutch assemblyoperably disposed between said first member and said second member, anda clutch actuator operable for applying a clutch engagement force onsaid friction clutch assembly, said clutch actuator including a screwcam having a threaded segment disposed within a chamber filled with amagnetorheological fluid and an electromagnetic coil arranged to varythe viscosity of said fluid in said chamber in response to an electriccontrol signal; and a controller for generating said electric controlsignal.
 23. The transfer case of claim 22 wherein said chamber isprovided between a housing and said threaded segment of said screw cam,and wherein said electromagnetic coil is mounted to said housing. 24.The transfer case of claim 22 wherein said controller establishes thevalue of said electric control signal based on a rotary speed differencebetween said first output shaft and said second output shaft, andwherein said control signal is operable to vary the viscosity of saidmagnetorheological fluid in said chamber for causing relative rotationbetween said first member and said screw cam which results in axialmovement of said screw cam relative to said friction clutch assembly.